Posts: 7
Joined: 4/23/2003 From: United States Status: offline
Maybe you all can help me with this. I have a 90 GT that should have had a 2.73 rear gear. I bought the car used from a lot so no previous owner to talk to. Since then I have found things that are not stock. The speedo is exactly ten mph off.(faster) so I got to looking at the rear end and noticed it had been into. I used this method to try and see if the gear ratio had been changed:
1) Jack up one rear wheel and chalk-mark the drive shaft and the 12 o'clock position on the tire. (Make sure the slack is out of the drive train)
2) Rotate the drive shaft 2 complete turns.
3) See where chalk mark on tire ends up.
A) Between 11 and 12 o'clock, ratio is 4.11
B) Between 12 and 1 o'clock, ratio is 3.78
C) Between 1 and 2 o'clock, ratio is 3.55
I wound up with C) 3.55. Now I went to Smokemup.com and used their gear ratio calculater:
Tire Diameter 25.63
Speed 60mph
RPM 2800
Result: 3.56 : 1 Gear Ratio
I thought I had something but I read somewhere else that the drive shaft would rotate about 3 1/2 times for it to be 3.55's. Thats a round and a half more than what my shaft turned. Now I have an AOD and I know the tranny couldn't be in overdrive for the RPM test but does it make any difference if it is in overdrive or drive on the chalk mark test?
Also does anybody have a better way of checking this without taking the rear cover off or does it look like the 3.55's are right?
Posts: 53
Joined: 12/27/2002 From: United States Status: offline
I don't know man sounds like with all the trouble your trying go thru ,you could of had you diff plate off and counted you teeth on the ring and pinion????
Posts: 352
Joined: 4/17/2003 From: New Jersey Status: offline
When i put my rear in with the AOD trans at a performance shop in pottstown PA, they told me that 3:73 are the popular gear for the AOD. But what you said and found out it just doesn't make sence.
Posts: 352
Joined: 4/17/2003 From: New Jersey Status: offline
Oh man that hurt. Im still young so i dont have to worry about that stuff yet. Good luck with your stang. Keep in touch, i'd like to know what you do with it. Ill try to get pictures up to show you what she looks like
Posts: 7
Joined: 4/23/2003 From: United States Status: offline
ECR5.0 I just bought the car a couple of months ago so I am just going over things and trying to replace some parts that are wore out. I don't know who the previous owner was so I can't ask what all has been changed or added to the car.
Some of the obvious stuff:
K&N filter kit
Aluminum heads
MAC shorty headers
Flowmaster exhaust
AOD (with shift kit)
3.55 rears (maybe)
All and all the car is in great shape inside and out. Also I thank you and MuDvAyNe for the good luck wishes. I will just have to wait until the end of summer for the new paint job.lol
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A raer gear question - 
